Move admin header partial to a component

This way it's easier to refactor it.

Note we're using `with_request_url` in the tests because the component
renders the locale switcher, which needs a URL in order to work. This
doesn't affect whether we're in the management section or not.

app/components/layout/admin_header_component.rb

@@ -0,0 +1,20 @@
+class Layout::AdminHeaderComponent < ApplicationComponent
+  attr_reader :user
+  delegate :namespace, :namespaced_root_path, :show_admin_menu?, to: :helpers
+
+  def initialize(user)
+    @user = user
+  end
+
+  private
+
+    def namespaced_header_title
+      if namespace == "moderation/budgets"
+        t("moderation.header.title")
+      elsif namespace == "management"
+        t("management.dashboard.index.title")
+      else
+        t("#{namespace}.header.title")
+      end
+    end
+end